This invention relates to ferrofluidic seals and, in particular, to apparatus that ameliorates the effect of seal stage bursting in high-vacuum environments.
Ferrofluidic rotary seals have been widely used in vacuum applications over the past 20 years. The basic structure of the seal comprises one or more magnets, a rotary shaft, pole pieces, and a housing. Additional parts may also be present as is known in the art. The magnets, the pole pieces and the rotary shaft form magnetic circuits with air gaps that occur between the pole pieces and the shaft. A ferrofluid is placed in the air gaps and forms a liquid O-ring rotary seal between the pole pieces and the rotary shaft. As used herein, a ferrofluid comprises magnetic particles coated with a surfactant that are suspended in a carrier liquid that may be water or oil. The magnetic particles are sufficiently small (approximately 10 nanometers) that they are colloidally suspended in the carrier liquid. A rubber O-ring at the radial interface usually provides a seal between the stationary parts, such as that between a pole piece and the housing. Seals with the above structure have been effectively used in a wide variety of applications, such as semi-conductor manufacturing, optical coating, rotary gas unions etc.
Due to the fact that a liquid forms the seal, the pressure capacity of a single seal stage is limited and is dependent upon the magnetic circuit design and the magnetization of the ferrofluid. Typically, one stage can withstand a 2-4 psi pressure difference across the stage without failure. Consequently, in applications that require a seal to withstand a pressure differential more than can be supported by a single stage, multiple seal stages are used.
A typical seal and bearing unit 100 with multiple seal stages is shown in sectional view in FIG. 1. The unit 100 comprises a non-magnetic housing 102 that surrounds a rotary shaft 104 fabricated from a magnetic material. The ferrofluid seal is comprised of magnetic pole pieces 106 and 108, magnet 110 and the shaft 104. The seal pieces form a magnetic circuit indicated schematically by dotted box 115. The pole pieces 106 and 108 extend close to, but do not touch, the shaft 104 to form small gaps between the pole pieces 106, 108 and shaft 104. The magnetic circuit 115 extends across gaps between pole pieces 106 and 108. A ferrofluid 112, located in the gaps is held in position by the magnetic field in the gaps. Rubber O-rings 130 and 132 seal the stationary pole pieces, 106 and 108, respectively, to the housing 102 to support the pressure differential between a low pressure (vacuum) area 101 and a high pressure (which may be atmospheric) area 103.
The seal unit 100 may also include a bearing assembly 128 that has one or more bearings 124 and 126. The unit is completed by a cover plate 118, fastened to an end of the housing 102 by clamping screws, of which screws 120 and 122 are shown in FIG. 1.
Although there are only two pole pieces 106 and 108, slots 116 are cut into the shaft to form multiple seal stages. Alternatively, slots may be cut into the faces of the pole pieces that oppose the shaft 104 to form the seal stages. At each seal stage the magnetic fluid forms a liquid O-ring, which provides a hermetic seal between the rotary shaft 104 and the stationary pole piece 106, 108. Thus, multiple seal stages are formed each of which can support a pressure differential. This arrangement is shown in greater detail in FIG. 2.
In FIG. 2, the shaft 104 has a plurality of slots 116 cut into its surface, leaving a plurality of ring-shaped teeth 250, 252, 256, etc. The teeth extend close to, but do not touch the inner surface 105 of pole piece 106. Because the magnetic field in concentrated in the gaps between the teeth 250, 252, 256 and inner surface 105 of pole piece 106, ferrofluid 112 is attracted to the gaps and forms a plurality of seal stages. Each of these seal stages will be referred to below by the numeral designation of the tooth that forms it. Interseal areas 254, 258, etc. exist between each seal stage.
During a pump down process in which a pressure differential is applied across the seal, the differential pressure across the first stage 250 facing the vacuum side of the seal is increased due to the vacuum. Once the differential pressure exceeds the pressure capacity of the first stage 250, the ferrofluid 112 at the first stage 250 is temporarily pushed out of the gap and the seal stage xe2x80x9cburstsxe2x80x9d to relieve some of the pressure differential across the stage 250. When the seal stage 250 bursts, the stage 250 allows part of the gas stored in the interstage area 254 between the first stage 250 and the second stage 252 to leak into the vacuum area 101, thereby reducing the gas pressure in the interstage area 254. Thus, the differential pressure across the first stage 250 is reduced while the pressure differential the second stage 252 is increased.
Eventually, the pressure differential across the second seal stage 252 will exceed the capacity of the seal stage and it too will burst, thereby decreasing the pressure differential across it and increasing the pressure differential across the first seal stage 250 and the third seal stage 256. Sometimes the increase in pressure differential caused by a seal stage bursting can increase the pressure differential across an adjacent stage causing it to burst also. A xe2x80x9ccascadexe2x80x9d effect results until a volume of gas is released into the low-pressure area. Such a process continues between the stages during the pump down process, until the differential pressure between vacuum area 101 and the atmospheric area 103 is approximately equally shared by a plurality of seal stages. Each time the first stage 250 bursts, the pressure in the vacuum area fluctuates and the gas in the interstage area is released into the vacuum area.
Typically, the aforementioned bursting phenomenon is not harmful during the pump down process because at this time, a processing job inside the vacuum chamber that requires a high vacuum has not been started. This processing job can be wafer processing in semiconductor industry, thin film coating in the optics component industry or some other conventional processing job that requires high vacuum. However, various factors, such as shaft rotation and pressure variations can cause a seal to burst after pump down. If seal stage bursting continues while the processing job proceeds, the resulting pressure fluctuations and release of gas into the vacuum chamber is not desirable. In particular, the larger the amplitude of the pressure fluctuation, the more deleterious the consequences to the processing job. A typical pump down profile is illustrated in FIG. 5 that shows the processing chamber pressure on the vertical scale versus time on the horizontal scale. As shown, with a conventional multiple stage ferrofluid seal, seal stage bursting can cause the pressure in a vacuum chamber to fluctuate over three orders of magnitude, for example, from 10xe2x88x927 Torr to 10xe2x88x925 Torr, then back to 107 Torr in a period of a few seconds as shown by the pressure spike 500.
Therefore, there is a need for a ferrofluid seal structure that minimizes the impact of seal stage bursting during the processing phase of a job.
In accordance with the principles of the invention, a reservoir is created between the first seal stage at the low-pressure side and its adjacent seal stage. The volume of the reservoir is relatively large compared to the volumes of the interstage areas between the other seal stages. In addition, a controlled leakage path bypasses the first stage from the reservoir to the low-pressure area. The leakage rate through the bypass path is controlled so that gas in the reservoir leaks to the low-pressure area relatively slowly; for example, the bypass path might equalize the pressure across the first stage over a five-minute interval.
Consequently, when a processing job is occurring, bursting will not occur at the first stage since the differential pressure across the first stage is much lower than its pressure capacity, due to the controlled leakage through the bypass path that reduces the pressure differential during the pump down process. When fluid bursting occurs at stages behind the second stage, such as the third, the fourth, or the fifth stage, the pressure in the vacuum area will not fluctuate because the second stage provides a seal. When the second stage bursts, the gas that is released is stored in the reservoir. Since the capacity of the reservoir is large relative to the interstage volume between the second and third stages, the first stage will not burst when the second stage bursts. The gas in the reservoir is then slowly released via the leakage path into the vacuum area. However, because the leakage rate is relatively slow, rapid fluctuations in the vacuum area are avoided and the small volume of gas released into the processing area can easily be handled by the pump down mechanism.
In one embodiment, the leakage rate through the bypass path is adjustable. For example, a needle valve or other adjustable leakage path can be used.
In another embodiment, the bypass path is routed through a seal pole piece. In yet another embodiment, the bypass path is routed through the teeth that form the seal stages.